Low compression 1 cylinder

It might have broken the valve spring on that #7 intake valve.   Easy to remove and check.  Over Revving is one of the few things that will fracture a valve spring.  If one broke it is probably prudent to change all 16, they were exposed to the same stresses.   This is just speculation because we don’t know if the engine hit 5,000 RPM or 10,000 RPM at this point.  If you are removing valve springs it is also prudent and the perfect time to replace the valve stem seals.   Edit, if your valve stem seals were new in those heads 2 years ago they are fine,  Stem seals usually age out after 15-20 years of use, not 2 years if quality parts were used.

You can tilt the head up so the exhaust is down and the intake ports are facing up.  Spray a fair amount of WD 40 or any thin oil into that intake port on #7.   I have used Diesel fuel, Laquer thinner or acetone for this test also.  They wont rust your valve but are thin and will leak out around a valve that is not seated.  If you want to test the exhaust roll the had over and try that side.   Let is sit at least 15 minutes.  It will usually seep right out if the valve is not sealed tight.  A very slow seep that only leaves a small stain but is not running out could be OK.  You can tap on the valve with the wood end of a hammer and see if it seals after tapping.   To have 70 PSI a valve would have a fast leak.  Has the engine been overheated and were any of the head bolts loose on tear down?   You can also test the valve with high pressure air, 100 PSI blowing on the back of the valve while spraying soapy water on the chamber side of the valve looking for leaks but water on cast iron rusts quickly.

Damn ok. Thanks for the help! I’ll let you know what I find.

To give good advice we need good information, your compression test is fact and good information, the leak down information can be very helpful if you know if it is coming out your carburetor or if it is coming out your dip stick or PCV valve.